One approach I've used a couple of times with success is to send calvary far out on one flank and ahead of the main line. As infantry lines get close but before they meet, race a unit or two of cavalry behind their line and ride parallel with his infantry line, but don't actually engage.

If you engage your calvary will most likely be slaughtered quickly but if you simply blaze by as fast as your cav will go, you can force the opponent to turn part of his line to face you to avoid an attack to the rear or force him to have a potential threat in his rear right before the two lines meet. The overall result is that the enemy infantry line will be fragmented by the time the firing begins.